This application addresses a major gap for medical treatment of radiation combined injuries (CI). Fluoroquinolones, like ciprofloxacin (CIP), are generally well-tolerated and commonly used for treating infections in the general population. The Food and Drug Administration (FDA) has recommended CIP for anthrax treatment, and it is maintained in the National Stockpile for specific human use in the event of a national emergency. We hypothesize that CIP exerts a radioprotective/therapeutic effect either indirectly through genotoxic stress response, or directly via ability to modify chromatin and/or inhibit eukaryotic topoisomerase (topo) II. CIP works by inhibiting bacterial DNA gyrase (bacterial topo II or topo IV), but has a slight inhibitory effects on eukaryotic topo II. Topo II catalyzes the double strand DNA break/religation reaction. Our rationale is based on: 1) mouse studies where CIP enhanced survival after 9.75 Gy total body irradiation (TBI) and CI mice and 2) increased DNA repair capacity in rhesus macaques that received 6.5 Gy TBI with CIP compared to TBI. Our long-term goals are to (1) clarify the pathologic mechanisms associated with combined injury (CI-radiation plus skin-wound trauma) and (2) assess the efficacy of selected antimicrobials for treating sepsis after CI. Our short-term goals are to 1) clarify the molecular mechanisms associated with apparent enhanced genomic stability conferred on TBI-CIP animals, and 2) to elucidate molecular pathways involved in increased survival after radiation injury (RI) and CI in B6D2F1 female mice. We will address these goals in three Specific Aims: Aim 1. Assess biological activities of CIP in human lymphoblastoid TK6 cells and a blood ex vivo pharmacodynamic assay, Aim 2. Measure CIP-molecular pharmacodynamics in mice in vivo by (a) quantify homologous genes and proteins for cell cycle, DNA repair, and apoptosis in hematopoietic, wound tissues following CI, and (b) assess DNA damage/ repair and apoptotic markers by FACS, and Aim 3. Discriminate changes in genomic stability post CI and CIP in hematopoietic samples for RI and CI by genomic challenge assay ex vivo. The R33 portion will be divided into 3 defined tasks: (a) radioprotective/therapeutic efficacy (b) mechanisms of action, and (c) genomic/proteonomic profiling. The increased understanding of immunomodulatory properties and activation of checkpoint pathways will facilitate greater utility of CIP, and of how quinolone antimicrobials might confer genomic stability, enhance wound healing, and improve survival.